We've added a new parameter type specifically for map scales, QgsProcessingParameterScale. Scale values are evaluated using self.parameterAsDouble, which return the map scale denominator (matching the standard in other parts of the QGIS API).
Scale parameters are displayed to users using the standard QgsScaleWidget, which includes the combo box of predefined scales and a shortcut button to match the current map scale. The addition of this new parameter type allows you to create Processing algorithms and models which require a scale choice at run time.